This application claims the priority of German patent document 195 22 539.2, the disclosure of which is expressly incorporated by reference herein.
1. Field of the Invention
The present invention relates to a solar cell having an emitter layer disposed on a base material, and a surface texture distinguished by rows of crisscrossing pointed ribs, and to a process for the fabrication of such a solar cell.
2. Background of the Related Art
Solar cells are components which convert sunlight or other light into electric energy. A solar cell usually comprises a semiconductor material, which is provided with a p-n junction. In the semiconductor, incident light generates positive-charged and negative-charged carriers which are separated by the p-n junction. The electric power generated in this matter is tapped via metal contacts on the n- and p-sides.
From the many known embodiments of solar cells, the subject matter of the present invention is confined to solar cells with a high degree of efficiency of &gt;20%. These electric components, also called "high-efficiency" solar cells, are preferably made of silicon. "High-efficiency" solar cells are distinguished, in particular, by the fact that losses due to the conversion of light into electricity are minimal. This is made possible by a number of measures, described in detail, for instance, by M. A. Green, "High-efficiency Silicon Solar Cells", TransTech Publications, Aedermannsdorf, Switzerland (1987).
The solar cell with the hitherto highest attainable efficiency is the so-called PERL ("Passivated emitter and rear, locally diffused") solar cell described, for example in Appl. Phys. Lett. 57(6), Jun. 8, 1990, pp. 602-604. The construction of [the, as] such known, "high performance solar cell" can be seen in FIG. 1. The PERL cell is provided with a textured surface S, which is coated with a layer of silicon dioxide S10.sub.2, due to its electric passivation and antireflex effect. Under the silicon dioxide layer is an emitter layer E, which is provided with regions of high n.sup.++ doping and of normal n.sup.+ doping. The emitter layer is formed in a three-dimensional configuration by selective etching in such a manner that the contour of its surface corresponds to the impression of many adjacently disposed square pyramids. The emitter layer is provided deeper and with higher doping only at sites where the emitter layer comes into contact with a metal contact strip M.
Due to the typical design of the characteristic surface texture of the emitter layer, on the one hand, the light penetrating through the emitter layer into the solar cell enters the cell with less losses and, on the other hand it cannot so easily leave the interior of the solar cell again. In addition to the antireflex effect of the top silicon dioxide layer, this layer also contributes to the electric passivation of the surface.
An essential aspect of this textured surface of the PERL cell is the two-step n-doping of the emitter, which in the area under the contacts of the metal bridges is high doped (and therefore low-ohmic) and is deeper than in the areas under the "inverted pyramids", where the emitter layer is doped weaker and runs flatter. (For the sake of completeness, it is noted that base contact electrodes BE are provided on the rear side of the solar cell facing the emitter layer.)
Two photomasking steps are required for the fabrication of PERL solar cells: an etching step to produce the riblike surface texture and a local diffusion step for placing the deep n.sup.++ diffusion areas at sites over which the metal contact bridges are disposed. In a third process step, the surface contour of the solar cell is coated with a full-surface n.sup.+ diffusion layer.
Despite its previously unattainable degree of efficiency, the two previously mentioned masking steps make fabrication of the PERL solar cell complicated and expensive.
Furthermore, the elongated metal contact bridges reduce the effective solar cell surface which absorbs light energy, thereby limiting its efficiency.
One object of the present invention is to provide an improved construction of a so-called PERL solar cell, which on the one hand, simplifies its fabrication and, on the other hand, further enhances its degree of efficiency.
Another object of the invention is to minimize the portion of surface of the solar cell which is covered by contacting elements, and therefore cannot contribute to the conversion of light.
These and other objects and advantages are achieved by the solar cell and fabrication process according to the invention, in which the solar cell has a grid-shaped surface texture provided with an n-doped emitter layer formed on a base material. In a first step of the fabrication process, a full-surface high-doped n.sup.++ doping layer is produced in a to-be-processed base material. Preferably, this n.sup.++ doping layer is placed in the top region of the base material by diffusion of the dopants. A subsequent selective etching process textures the n.sup.++ doping layer using an etching mask on its surface, so as to create a multiplicity of intersecting rows of ribs having a pointed cross section, and whose top sections are composed of the n.sup.++ doped doping layer and their respective bottom parts of the base material (see FIG. 2).
In this manner, it is achieved that the top edges of the rows of ribs are composed of high-doped material. Thus in an overall view of all the crisscrossing rib edges on the top side of emitter layer, a low-ohmic grid covering the entire cell surface is formed on the top side of the emitter layer.
A particular advantage of the grid-emitter structure according to the invention, (also referred to herein as a "GE" structure) is that by placing, a fine contact wire grid on the GE structure, electric contacting of the emitter of the solar cell can be accomplished, thereby reducing to a minimum the shading effect caused by the filigree design of the contact grid. Moreover, the invented process also obviates the second, expensive photomasking step required in the conventional process, thereby lowering fabrication costs.
In order to create the parts of the surface which lie between the high-doped top edges of the ribs and are able to contribute to converting electromagnetic radiation into electric energy due to the etching procedure down to the base material, the entire surface texture of the invented solar cell can be doped with n.sup.+ dopants. In this manner, a two-step doped emitter is obtained, which is provided with regions of highest doping only under the top edges of the rows of ribs with the doping diminishing with rib depth. This two-step manner of doping favorably influences recombination in the emitter.
By selecting different mask configurations for the etching process it is possible to control the configuration of the rib rows. Furthermore, the invented process permits fabricating a solar cell whose one surface is provided with both emitter and base connections.
According to the present invention, the following steps are used to fabricate a solar cell of this type:
First, the whole surface of a base material is doped with p.sup.+ dopants, preferably by diffusion doping. Subsequently, the surface predoped in this manner is doped once more with n.sup.++ dopants. The doping profile is selected such that the n-dopants penetrate less deeply into the base material than the p-dopants. To produce a desired surface texture, an etching mask is employed in a subsequent selective etching process in order to obtain a multiplicity of intersecting rows of pointed ribs. The etching mask configuration is selected so that the rows of ribs in the peripheral surface region are higher than the internal rows of ribs enclosed by the peripheral rows; thus, the top section of the higher rows of ribs are composed of the n.sup.++ doped doping layer and the top sections of the internal rows of ribs are composed of p.sup.+ -doped doping layer.
The invented process also makes it possible to obtain regions of varying doping characteristics on one and the same surface which, moreover, differ from each other with regard to their relative level. Thus, the emitter level which is composed of the top edges of the higher rows of ribs project above the base level which is composed of the top edges of the lower rows of ribs. As the following descriptions accompanying the figures show, the height of the individual rows of ribs can be varied by a suitable selection of the masks employed during the etching procedure.
Compared to the known PERL solar cell, the invented improvement is provided with rows of ribs whose top pointed region is doped n.sup.++ and bottom region is composed of the base material.